High frequency plasma generator and high frequency plasma generating method

ABSTRACT

An object is to provide a high-frequency plasma generating apparatus and process which can further advance uniformity of the thickness of a film on a substrate with a large area in comparison with conventional apparatuses. In a reaction chamber ( 1 ), a ground electrode ( 3 ) is disposed, and a discharge electrode ( 2 ) is disposed opposite to the ground electrode ( 3 ). A substrate ( 4 ) as a processing object is placed in close contact with the ground electrode ( 3 ). A high-frequency voltage is applied to the discharge electrode ( 2 ) so as to generate plasma between the ground electrode and the discharge electrode. An RF electric power supply ( 15 ) generates a first high-frequency voltage, and outputs the generated voltage on feeding points ( 9 ) disposed on a lateral portion of the discharge electrode ( 2 ). An RF electric power supply ( 16 ) generates a second high-frequency voltage, and outputs the generated voltage on feeding points ( 9 ) disposed on another lateral portion of the discharge electrode ( 2 ). Here, the second high-frequency voltage has the same frequency as that of the first high-frequency voltage and has a phase which varies with a low-frequency signal, which is modulated by a predetermined modulation signal.

TECHNICAL FIELD

The present invention relates to high-frequency plasma generatingapparatuses and high-frequency plasma generating processes, which can beapplied to formation of films of semiconductors useful in solar cells,thin-film transistors, or the like, such as amorphous silicon,microcrystalline silicon, polycrystalline thin-film silicon, and siliconnitride, or which can be applied to etching of semiconductor films.

BACKGROUND ART

FIG. 5 shows an example of a conventional high-frequency plasmagenerating apparatus. FIG. 6 is a cross-sectional view showing areaction chamber 1 of the same apparatus. The high-frequency plasmagenerating apparatus shown in these figures can be used for productionof a thin film of amorphous silicon semiconductor for solar cells.

The inside of the reaction chamber 1 shown in FIG. 5 and FIG. 6 isequipped with a ladder-shaped electrode 2 as a discharge electrode and aground electrode 3. The reaction chamber 1 is made gastight, to which agas supply pipe and an exhaust pipe (both of which are not shown in thedrawings) are open at appropriate positions respectively. Through thegas supply pipe, which communicates with a gas supply source, a gas forfilm formation is introduced into the reaction chamber 1. The exhaustpipe communicates with the suction side of a vacuum pump. The reactionchamber 1 can be evacuated to an internal pressure of about 1 ×10⁻⁶ Torrusing this vacuum pump.

The ground electrode 3 and the ladder-shaped electrode 2 are disposedopposite to each other at a predetermined distance (for example, adistance of 20 mm). The ground electrode 3 is equipped with a mechanismfor holding a glass substrate 4 as a processing object and has a heaterbuilt in so as to heat the glass substrate 4. The ladder-shapedelectrode 2 needs to be larger than the glass substrate 4 and is arectangle with the dimensions 1.25 m by 1.55 m when the glass substrate4 is a rectangle with the dimensions 1.1 m by 1.4 m.

A gas diffusion port of the gas supply pipe is open desirably behind theladder-shaped electrode 2 (i.e., on the opposite side to the glasssubstrate 4). Gas is supplied preferably in parallel from severalpositions.

As shown in FIG. 5, the ladder-shaped electrode 2 is formed byassembling a plurality of parallel longitudinal electrode rods 6 and apair of transverse electrode rods 7 and 8 into the form of a lattice,and the ladder-shaped electrode 2 is disposed in parallel with andopposite to the glass substrate 4, which is held by the ground electrode3. Each of transverse electrode rods 7 and 8 of the ladder-shapedelectrode 2 is provided with eight feeding points 9. Feeding points 9 ofthe transverse electrode rod 7 are individually connected to an electricpower divider 11, and feeding points 9 of the transverse electrode rod 8are individually connected to an electric power divider 12. The electricpower dividers 11 and 12 are connected to impedance matchers 13 and 14,respectively, by coaxial cables. The impedance matchers 13 and 14 areconnected to RF (high-frequency) electric power supplies 15 and 16,respectively. The RF electric power supply 15 is connected to an outputportion of an oscillator 17. The RF electric power supply 16 isconnected via a phase modulator 21 to the output portion of theoscillator 17. The phase modulator 21 is a circuit which modulates thephase of an output signal S from the oscillator 17 according to outputfrom a sine wave (or triangle wave) oscillator 18, and outputs themodulated signal to the RF electric power supply 16. Here, the outputamplitude of the oscillator 18 is constant, and therefore, the phaseshift Δθ of modulation by the phase modulator 21 is constant.

With the above structure, the glass substrate 4 on which an a-Si thinfilm is to be formed is placed on the ground electrode 3 which is set ata temperature of 200° C., for example, then SiH₄ gas is introduced at aflow rate of 2 slm, for example, from the gas supply pipe, and theexhaust rate of the vacuum pump system which is connected to the vacuumexhaust pipe is regulated, so as to adjust the pressure inside thereaction chamber 1 to, for example, 40 Pa (300 mTorr). Then, ahigh-frequency signal of 60 MHz generated by the oscillator 17 isamplified using the RF electric power supplies 15 and 16, and is appliedto the transverse electrode rods 7 and 8 of the ladder-shaped electrode2 through the impedance matchers 13 and 14 and the electric powerdividers 11 and 12. This operation generates plasma between the glasssubstrate 4 and the ladder-shaped electrode 2. At this point, theimpedance matchers 13 and 14 are adjusted so that the high-frequencyelectric power can be efficiently supplied to the plasma generatingpart. In the plasma, SiH₄ is decomposed, and an a-Si film is formed onthe surface of the glass substrate 4. An a-Si film with a requiredthickness can be formed by continuing this film forming operation inthis condition for, for example, about 5 to 10 minutes.

However, there is a drawback to the above-described conventionalhigh-frequency plasma generating apparatus in that it is difficult touniformly form a large area film. This is because a standing wave isgenerated mainly due to a reflected wave which occurs at an end of anelectrode or the like, since the wavelength of the high-frequency waveis about on the same order as the sizes of the electrodes 2 and 3. Forexample, the wavelength for 60 MHz on the ladder-shaped electrode wouldbe about 3 m. Although the wavelength for 60 MHz in a vacuum is 5 m, thewavelength in plasma is shortened due to increase in capacitance. Thiswavelength of 3 m gives a ¼ wavelength of about 0.75 m as opposed to theelectrode length of 1.25 m, creating maximum and minimum voltage pointson the electrode. Therefore, plasma becomes nonuniform following thevoltage distribution on the electrode, causing as a result a problem inthat a film is formed nonuniformly.

In order to solve such a problem, there are apparatuses as in JapanesePatent Application No. 2001-133830, and there are apparatuses asdisclosed in Japanese Patent Application, First Publication (Kokai), No.2001-274099.

Since an apparatus according to Japanese Patent Application No.2001-133830 employs a method of high-speed switching between asingle-frequency plasma and a double-frequency plasma, plasma changesdiscontinuously, and a strong plasma is formed in the vicinity offeeding points. Accordingly, there is a limit to obtaining uniformity inthe distribution of film thickness, and when a strong plasma exists, thepossibility arises that generation of nanoclusters degrades the qualityof the film.

On the other hand, an apparatus as disclosed in Japanese PatentApplication, First Publication (Kokai), No. 2001-274099 is an apparatusas shown in FIG. 5 with which the phase of the high-frequency applied tothe transverse electrode rods 7 and 8 is periodically varied. That is,the phase of output from the RF electric power supply 16 is periodicallyvaried with respect to the phase of output from the RF electric powersupply 15 (see FIG. 4 and paragraphs 0091 to 0096 of that publication).

DISCLOSURE OF INVENTION

The apparatuses disclosed in Japanese Patent Application, FirstPublication (Kokai), No. 2001-274099 are to reduce nonuniformity of thethickness of a film formed by an apparatus as shown in FIG. 5. However,nonuniformity as explained in the following still remains in thelongitudinal direction (i.e., in the direction along the electrode rod6). That is, when the phase difference Δθ in output of the RF electricpower supply 16 from output of the RF electric power supply 15 isperiodically (10 KHz) varied in the range of −90° to +90°, the thicknessof part of the film becomes large on the central part of the glasssubstrate 4 as shown in FIG. 7A. When the phase difference Δθ isperiodically varied in the range of −135° to +135°, the thickness ofpart of the film becomes large as shown in FIG. 7B, slightly outside thearea shown in FIG. 7A. When the phase difference Δθ is periodicallyvaried in the range of −180° to +180°, the thickness of part of the filmbecomes large on the edges of the glass substrate 4 as shown in FIG. 7C.The actual distribution of the thickness of a formed film is as shown inFIG. 8, in which the state of the distribution is seen. FIG. 7Acorresponds to FIG. 8E, FIG. 7B corresponds to FIG. 8F, and FIG. 7Ccorresponds to FIG. 8G. When the phase modulation angle was fixed asabove, it was difficult to uniformly distribute the thickness of a film.Furthermore, in the distribution of the thickness of a film formed witha fixed phase which is shifted by +45° without phase modulation, thereis a portion having a large film thickness extending up to the upperpart of the substrate as shown in FIG. 8B. This distribution of filmthickness does not correspond to that in FIG. 8D, which shows a resultwith the same phase shift of 45° and with phase modulation. Thus, it hasbecome clear that a portion where a high voltage is applied does notalways correspond with a portion which has a large film thickness withphase modulation.

The present invention was conceived under the above circumstances, andan object of the present invention is to provide a high-frequency plasmagenerating apparatus and a high-frequency plasma generating processwhich can further advance uniformity of the thickness of a film on asubstrate with a large area in comparison with the above-describedconventional apparatuses.

The present invention was achieved in order to solve the above problems,and according to the present invention, in a high-frequency plasmagenerating apparatus having a reaction chamber in which a groundelectrode is disposed and a discharge electrode is disposed opposite tothe ground electrode, so that a substrate as a processing object will beplaced in close contact with the ground electrode, and a high-frequencyvoltage will be applied to the discharge electrode so as to generateplasma between the ground electrode and the discharge electrode,

the high-frequency plasma generating apparatus comprises:

a first high-frequency generator which generates a first high-frequencyvoltage,

a first electric power feeder which applies the first high-frequencyvoltage to a feeding point disposed on a lateral portion of thedischarge electrode,

a second high-frequency generator which generates a secondhigh-frequency voltage, and

a second electric power feeder which applies the second high-frequencyvoltage to a feeding point disposed on another lateral portion of thedischarge electrode,

wherein the second high-frequency voltage has the same frequency as thatof the first high-frequency voltage and has a phase which varies with alow-frequency signal, which is modulated by a predetermined modulationsignal.

Alternatively, according to the present invention, in a high-frequencyplasma generating apparatus having a reaction chamber in which a groundelectrode is disposed and a discharge electrode is disposed opposite tothe ground electrode, so that a substrate as a processing object will beplaced in close contact with the ground electrode, and a high-frequencyvoltage will be applied to the discharge electrode so as to generateplasma between the ground electrode and the discharge electrode,

the high-frequency plasma generating apparatus comprises:

a high-frequency oscillator which generates a high-frequency signal,

a first high-frequency generator which amplifies the high-frequencysignal from the high-frequency oscillator to yield a firsthigh-frequency voltage, and which outputs the first high-frequencyvoltage,

a first electric power feeder which applies the first high-frequencyvoltage to a feeding point disposed on a lateral portion of thedischarge electrode,

a low-frequency oscillator which generates a low-frequency signal whichis modulated by a predetermined modulation signal,

a phase modulator which modulates the phase of the high-frequency signalfrom the high-frequency oscillator with the low-frequency signal,

a second high-frequency generator which amplifies the high-frequencysignal modulated by the phase modulator to yield a second high-frequencyvoltage, and which outputs the second high-frequency voltage, and

a second electric power feeder which applies the second high-frequencyvoltage to a feeding point disposed on another lateral portion of thedischarge electrode.

Alternatively, according to the present invention, in either of theabove high-frequency plasma generating apparatuses, the dischargeelectrode is a ladder-shaped electrode formed by disposing a pluralityof longitudinal electrode rods between two transverse electrode rods,and the feeding point is disposed on the transverse electrode rods.

In addition, according to the present invention, in a high-frequencyplasma generating process in which a substrate as a processing object isplaced in close contact with a ground electrode, which is disposed in areaction chamber in which a discharge electrode is disposed opposite tothe ground electrode, and a high-frequency voltage is applied to thedischarge electrode so as to generate plasma between the groundelectrode and the discharge electrode,

the high-frequency plasma generating process comprises:

applying a first high-frequency voltage to a feeding point disposed on alateral portion of the discharge electrode, and

applying a second high-frequency voltage to a feeding point disposed onanother lateral portion of the discharge electrode, the secondhigh-frequency voltage having the same frequency as that of the firsthigh-frequency voltage and having a phase which varies with alow-frequency signal, which is modulated by a predetermined modulationsignal.

In addition, according to the present invention, in a process forcleaning a high-frequency plasma generating apparatus having a reactionchamber in which a ground electrode is disposed and a dischargeelectrode is disposed opposite to the ground electrode, so that asubstrate as a processing object will be placed in close contact withthe ground electrode, and a high-frequency voltage will be applied tothe discharge electrode so as to generate plasma between the groundelectrode and the discharge electrode,

the process for cleaning the high-frequency plasma generating apparatuscomprises:

introducing a halogen compound such as NF₃, CF₄, CCl₄, SF₆ into thereaction chamber,

applying a first high-frequency voltage to a feeding point disposed on alateral portion of the discharge electrode, and

applying a second high-frequency voltage to a feeding point disposed onanother lateral portion of the discharge electrode, the secondhigh-frequency voltage having the same frequency as that of the firsthigh-frequency voltage and having a phase which varies with alow-frequency signal, which is modulated by a predetermined modulationsignal.

By the high-frequency plasma generating apparatus and in thehigh-frequency plasma generating process according to the presentinvention, a first high-frequency voltage is applied to a feeding pointdisposed on a lateral portion of the discharge electrode, and a secondhigh-frequency voltage is applied to a feeding point disposed on anotherlateral portion of the discharge electrode, the second high-frequencyvoltage having the same frequency as that of the first high-frequencyvoltage and having a phase which varies with a low-frequency signal,which is modulated by a predetermined modulation signal. Therefore, thehigh-frequency plasma generating apparatus and high-frequency plasmagenerating process according to the present invention are more effectivein achieving uniformity in the thickness of a film formed on a substratehaving a large area than conventional apparatuses.

Accordingly, an apparatus according the present invention when used toproduce a thin film of amorphous silicon semiconductor for an electriccell, for example, contributes greatly to improvement in the performanceof the electric cell since the thickness of the film can be madeuniform. In addition, cutting performance of laser in a laser etchingstep in a process can be greatly improved, which also contributes toimprovement in the performance of the electric cell.

In addition, since the film thickness can be made uniform according tothe present invention, a pattern such as interference fringes does notappear, and therefore the present invention is advantageous in improvingthe appearance of products.

Furthermore, the process for cleaning a high-frequency plasma generatingapparatus according to the present invention is effective in allowingthe apparatus to self-clean without causing overetching. Moreover, sinceuniformity in cleaning is improved, the process has effects ofshortening time for cleaning and contributing to minimizing influence onthe distribution of the formed film before and after cleaning.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a structure of a high-frequency plasmagenerating apparatus according to a first embodiment of the presentinvention.

FIG. 2 is a diagram showing a signal for phase modulation generated byan oscillator 20 in the same embodiment.

FIG. 3 is a diagram to be used in explaining effects of the sameembodiment.

FIG. 4 is a diagram showing results of measuring the thickness of thethin film of semiconductor shown in FIG. 3.

FIG. 5 is a diagram showing an example of a structure of a conventionalhigh-frequency plasma generating apparatus.

FIG. 6 is a cross-sectional view of a reaction chamber 1 in aconventional high-frequency plasma generating apparatus.

FIG. 7 shows diagrams to illustrate problems with a conventionalhigh-frequency plasma generating apparatus.

FIG. 8 shows diagrams to illustrate problems with a conventionalhigh-frequency plasma generating apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below byreferring to the drawings. FIG. 1 is a block diagram showing a structureof a high-frequency plasma generating apparatus according to thisembodiment. The same reference numerals as those in FIG. 5 are assignedin FIG. 1 to the parts which are structurally the same as those in FIG.5, and explanations of such parts are omitted.

The high-frequency plasma generating apparatus according to thisembodiment differs from the apparatus as shown in FIG. 5 in that anoscillator 20 and a phase modulator 21 deal with not only simpletriangle waves or the like, but also arbitrarily modulated waveforms.The oscillator 20 is a circuit which generates a signal in a trianglewave with a frequency of 20 KHz as shown in FIG. 2, the signal havingbeen subjected to amplitude modulation using a triangle wave with afrequency of 1 KHz. The phase modulator 21 is a circuit which modulatesthe phase of an output signal S (60 MHz) from an oscillator 17 using theoutput from the oscillator 20, and outputs the modulated signal to an RFelectric power supply 16. That is, as shown in FIG. 2, the phasemodulator 21 changes the phase of the signal S by +110° and outputs thechanged signal to the RF electric power supply 16 when the peak value ofoutput from the oscillator 20 is a positive maximum value, and the phasemodulator 21 changes the phase of the signal S by −110° and outputs thechanged signal to the RF electric power supply 16 when the peak value ofoutput from the oscillator 20 is a negative maximum value. The phasemodulator 21 changes the phase of the signal S by +20° and outputs thechanged signal to the RF electric power supply 16 when the peak value ofoutput from the oscillator 20 is a positive minimum value, and the phasemodulator 21 changes the phase of the signal S by −20° and outputs thechanged signal to the RF electric power supply 16 when the peak value ofoutput from the oscillator 20 is a negative minimum value.

The high-frequency signal S of 60 MHz with a phase modulated by thephase modulator 21 as described above is amplified by the RF electricpower supply 16, and is output on a transverse electrode rod 8 of theladder-shaped electrode 2 via an impedance matcher 14 and an electricpower divider 12. Such a structure allows the position where the plasmaturns back to fluctuate instead of being fixed since the phasemodulation angle of the high-frequency signal applied to the transverseelectrode rod 8 varies over time. Accordingly, concentration of plasmacan be avoided, and uniform plasma can be realized.

FIG. 3 shows a distribution of the thickness of a thin film of amorphoussilicon semiconductor formed by a high frequency plasma generatingapparatus according to the above embodiment. This figure can be regardedas showing a distribution of the film thickness obtained by continuouslychanging conditions through FIGS. 8A, 8B, 8C, 8D, 8E, and 8F so as tomake the distribution of the film thickness uniform in a time-averagedsense. As is clear from this figure, uniformity of a thin film isgreatly improved by carrying out phase modulation according to theprocess as shown in FIG. 2 in comparison with that of conventionalprocesses. FIG. 4 shows results of measuring the thickness of the thinfilm of semiconductor shown in FIG. 3.

Although both waveforms of oscillation and modulation by the oscillator20 are triangle waves according to the above embodiment, they are notlimited to triangle waves, and they may be sine waves or square waves.The frequency of the signal (20 KHz) and the frequency of the modulationwaveform (1 KHz) of the oscillator 20 are only for examples, and are notlimited to these frequencies.

Incidentally, in a high-frequency plasma generating apparatus, a siliconfilm may be deposited on the inside of a reaction chamber 1, and some ofthe silicon film may fall or produce silicon powder in the gas phase, asformation of amorphous silicon films or the like is repeatedcontinuously. If such silicon powder adheres to a glass substrate 4during formation of a film, a defective product would be produced.

Therefore, self-cleaning as described below is carried out in thisembodiment. That is, operation of film formation by the high-frequencyplasma generating apparatus is stopped temporarily, NF₃ (nitrogentrifluoride) to work as an etching gas having strong reactivity isintroduced into the reaction chamber 1, and a high-frequency voltage(with one side having undergone phase modulation) of 60 MHz, which isthe same as that used for the above-described film formation, is appliedto each of the transverse electrode rods 7 and 8 of the ladder-shapedelectrode 2. By doing this, NF₃ becomes plasma and is decomposed, andthe inside of the reaction chamber 1 is etched by F (fluorine) radicalsproduced by the decomposition, whereby the silicon film or the siliconpowder is gasified and removed as SiF₄ (silicon fluoride) as in thefollowing formula.F radial+Si→SiF₄

As opposed to a conventional apparatus which has a problem in thatoveretching (corrosion of metal part by F radical) occurs in some placesin the above case due to nonuniform plasma, the above embodiment allowsself-cleaning without overetching since plasma can be generateduniformly.

1. A high-frequency plasma generating apparatus having a reactionchamber in which a ground electrode is disposed and a dischargeelectrode is disposed opposite to the ground electrode, so that asubstrate as a processing object will be placed in close contact withthe ground electrode, and a high-frequency voltage will be applied tothe discharge electrode so as to generate plasma between the groundelectrode and the discharge electrode, the high-frequency plasmagenerating apparatus comprising: a first high-frequency generator whichgenerates a first high-frequency voltage, a first electric power feederwhich applies the first high-frequency voltage to a feeding pointdisposed on a lateral portion of the discharge electrode, a secondhigh-frequency generator which generates a second high-frequencyvoltage, and a second electric power feeder which applies the secondhigh-frequency voltage to a feeding point disposed on another lateralportion of the discharge electrode, wherein the second high-frequencyvoltage has the same frequency as that of the first high-frequencyvoltage and has a phase which varies with a low-frequency signal, whichis modulated by a predetermined modulation signal.
 2. A high-frequencyplasma generating apparatus having a reaction chamber in which a groundelectrode is disposed and a discharge electrode is disposed opposite tothe ground electrode, so that a substrate as a processing object will beplaced in close contact with the ground electrode, and a high-frequencyvoltage will be applied to the discharge electrode so as to generateplasma between the ground electrode and the discharge electrode, thehigh-frequency plasma generating apparatus comprising: a high-frequencyoscillator which generates a high-frequency signal, a firsthigh-frequency generator which amplifies the high-frequency signal fromthe high-frequency oscillator to yield a first high-frequency voltage,and which outputs the first high-frequency voltage, a first electricpower feeder which applies the first high-frequency voltage to a feedingpoint disposed on a lateral portion of the discharge electrode, alow-frequency oscillator which generates a low-frequency signal which ismodulated by a predetermined modulation signal, a phase modulator whichmodulates the phase of the high-frequency signal from the high-frequencyoscillator with the low-frequency signal, a second high-frequencygenerator which amplifies the high-frequency signal modulated by thephase modulator to yield a second high-frequency voltage, and whichoutputs the second high-frequency voltage, and a second electric powerfeeder which applies the second high-frequency voltage to a feedingpoint disposed on another lateral portion of the discharge electrode. 3.A high-frequency plasma generating apparatus according to claim 1 or 2,wherein the discharge electrode is a ladder-shaped electrode formed bydisposing a plurality of longitudinal electrode rods between twotransverse electrode rods, and the feeding point is disposed on thetransverse electrode rods.
 4. A high-frequency plasma generating processin which a substrate as a processing object is placed in close contactwith a ground electrode, which is disposed in a reaction chamber inwhich a discharge electrode is disposed opposite to the groundelectrode, and a high-frequency voltage is applied to the dischargeelectrode so as to generate plasma between the ground electrode and thedischarge electrode, the high-frequency plasma generating processcomprising: applying a first high-frequency voltage to a feeding pointdisposed on a lateral portion of the discharge electrode, and applying asecond high-frequency voltage to a feeding point disposed on anotherlateral portion of the discharge electrode, the second high-frequencyvoltage having the same frequency as that of the first high-frequencyvoltage and having a phase which varies with a low-frequency signal,which is modulated by a predetermined modulation signal.
 5. A processfor cleaning a high-frequency plasma generating apparatus having areaction chamber in which a ground electrode is disposed and a dischargeelectrode is disposed opposite to the ground electrode, so that asubstrate as a processing object will be placed in close contact withthe ground electrode, and a high-frequency voltage will be applied tothe discharge electrode so as to generate plasma between the groundelectrode and the discharge electrode, the process for cleaning thehigh-frequency plasma generating apparatus comprising: introducing ahalogen compound such as NF₃, CF₄, CCL₄, SF₆into the reaction chamber,applying a first high-frequency voltage to a feeding point disposed on alateral portion of the discharge electrode, and applying a secondhigh-frequency voltage to a feeding point disposed on another lateralportion of the discharge electrode, the second high-frequency voltagehaving the same frequency as that of the first high-frequency voltageand having a phase which varies with a low-frequency signal, which ismodulated by a predetermined modulation signal.